High Intensity Discharge (HID) lamps are widely used in airports, parking garages and on public streets to provide wide-area ambient lighting. Although recent electronic ballast designs can deliver better efficiency, improved power factor and longer bulb life than the standard magnetic ballast, most of the HID lamps currently in use employ magnetic ballasts, owing to their low cost, familiar characteristics and their decades-old domination of the HID and fluorescent lighting markets.
Given the ever increasing demands to improve energy efficiency of existing buildings and public structures, there is a strong financial incentive to retrofit existing HID lighting installations with modern, dimmable lighting systems. However, in many installations it is not economical to change out the existing light fixtures and ballasts, creating the need for a lighting controller that can reliably and accurately dim entire banks of existing magnetically ballasted HID lamps at the contactor control point.
Given that standard HID lamps and magnetic ballasts were not originally designed to be dimmable, this effort represents a complex engineering task. Several commercial examples have been developed to date, with varying degrees of success. Common problems with HID dimmers include flicker, line harmonics, poor power factor, reliability, and arc extinction during the dimming process.
A number of approaches have been taken to control the light intensity of a fluorescent lamp or a HID lamp. Some use pulse width modulation of an inverter drive, or change the supply voltage to the rectifying circuit for the inverter DC link voltage.
One approach uses a phase controlled dimmable ballast for a fluorescent lamp. A small portion of the phase of the input supply voltage is removed, and the amount of phase removed is used to generate a switching signal that controls the frequency of the electronic ballast and thus the light output.
In another version of an automatic light dimmer for gas discharge lamps, the apparatus applies full power to the primaries when the lights are first turned on for a pre-selected time period, thus ensuring all the lamps in the system are lit. After the pre-selected time period has passed, the apparatus automatically dims the lamps and maintains them in the dimmed state.
There are electronic ballasts with illumination control, such as U.S. Pat. No. 6,172,466 B1, known as phase-control dimmable ballast, which suppresses a portion of the supply voltage in each half cycle. This circuit interpolates the selected illumination level, adjusting conduction times in the solid-state switch element. This process affects the power factor of the ballast and contributes a degree of harmonic distortion to the power line. This system, which interacts with internal elements of the electronic ballast, cannot be built as an element that is external to the ballast because it requires structural changes in the system of common electronic ballasts.
Also well-known are the electronic ballasts that have a certain number of operating taps offered at their power terminals to set the lighting intensity. With this system, the different lighting taps can be wired to a multiple switch. The disadvantage is that the lighting changes suddenly and power cables must be added to the lighting circuit. The change in the illumination level is based on a circuit that, like the previous technique described, affects the conduction times in a solid-state switching system.
Even with these ballasts, patents and the patents cited in them there still remains a need for a simple and reliable means for providing dimming control for electronic and magnetic ballasts for fluorescent or HID lamps and which overcomes or at least minimizes many of the previously mentioned problems.